The choice of lighting
is one the most important decisions to make when setting up
a reef tank. The light fixtures and related equipment are
some of the more expensive pieces of equipment both at initial
setup up as well as in their contribution to daily operating
costs. In addition to being necessary for the photosynthetic
organisms we keep in our aquariums, light also provides the
visual element of color. From talking to aquarists and perusing
the various reef-related bulletin boards, it has been my experience
that lighting and color are often a very misunderstood aspect
of aquarium keeping. Given that lighting and color are important
in the functional and aesthetic elements of reefkeeping, I
feel it is important that hobbyists have a good understanding
of light and color. The purpose of this series of articles
is to provide beginning and intermediate reef aquarists with
a comprehensive understanding of lighting concepts and terminology,
and the ability to understand and comprehend lighting related
discussions and data. The information will be presented in
a series of short columns focusing on a few concepts at a
time and build to a comprehensive understanding of light,
especially as it relates to reef aquariums.

Light is a form of energy, and to understand light we begin
with the electromagnetic spectrum (Figure 1: http://www.lbl.gov/MicroWorlds/.../EMSpec2.html)
which is basically a grouping of all electromagnetic radiation
arranged according to the amount of energy contained in the
radiation. Visible light is a part of this electromagnetic
spectrum that creates the sensation of light when it falls
on the human eye.

Figure 1. The electromagnetic spectrum.

The properties of all electromagnetic radiation can be described
by three inter-related terms. These are wavelength,
frequency and energy. Since light is a part
of this spectrum, it too can be described by these terms.
Hence, it is important to understand these terms as a first
step towards understanding light.

1) Wavelength:

Simplistically, we can think of light
traveling as a wave. A typical wave form (e.g., ripples on
the surface of water) has crests (or peaks) and troughs (or
valleys). The distance between two consecutive peaks (or troughs)
is called the wavelength, and is denoted by the Greek letter
λ
(lambda). Because the wavelength is a measure of distance,
it is measured in units of length (meters). Since these wavelengths
of visible light can be quite small, they are measured in
nanometers(nm) where 1 nm = 1 billionth of
a meter (10-9
meters). The wavelength of visible light is between
400-700nm. Incidentally, these also happen to be the
majority of wavelengths of light that are relevant to photosynthesis.
The combined effect of the complete range of radiation between
400-700nm appears as white light to the human eye. Radiation
with a wavelength of 400 nm generates a response in
the human eye that makes it perceived as violet, while
radiation with a wavelength of 700nm appears red.
The different colors of the rainbow (ROYGBV - red, orange,
yellow, green, blue and violet) are arranged in descending
order of their wavelength. Roughly, we can break down the
various colors into wavelength bands as follows:

Violet - 400
to 440nm

Blue - 440 to
490nm

Green - 490 to
540nm

Yellow - 540
to 590nm

Orange - 600
to 650nm

Red - 650 to
700nm

Radiation below 400 nm wavelength is called ultraviolet (UV)
radiation, and is typically divided into three segments: UV-A
(400-315nm), UV-B (315-280nm) and UV-C (280-100nm). UV
radiation is not visible to the human eye, but it can have
a damaging impact on humans (as well as corals). The UV-A
segment, the most common in sunlight, overlaps slightly with
the shortest wavelengths in the visible portion of the spectrum.
UV-B is effectively the most destructive UV radiation from
the sun, because it penetrates the atmosphere and can injure
biological tissues. UV-C radiation from the sun would cause
even more injury, but it is absorbed by the atmosphere, so
it almost never reaches the Earth's surface.

Infrared (IR) radiation has slightly longer wavelengths
than visible light. The IR region of the electromagnetic spectrum
is also divided into three segments: IR-A (780-1400 nm), IR-B
(1400-3000 nm) and IR-C (3000-10600 nm). Infrared radiation
is thermal and is felt as heat.

2) Frequency:

The number of waves
that pass a given point in space during a specified time interval
is the light's frequency; consequently, frequency is a time
based unit. Frequency carries the units "per second,"
but we use a special term for the unit called - Hertz (Hz),
where 1 Hz corresponds to 1 wave/second, so 50 Hz would mean
50 waves/second.

As seen in the figure above, the wavelength
and frequency are related to each other. If we take any two
points on the waveform labeled "start" and "end,"
and count the number of waves in between, we can easily see
that we will have more waves if the wavelength is smaller.
More waves imply that the frequency will be higher. Thus wavelength
and frequency are inversely related: the shorter
the wavelength of the wave, the higher the frequency of the
wave.

Since all the waves travel at the same speed - the speed
of light - the relationship between wavelength and frequency
is determined by the following formula:

Wavelength = speed of light / frequency

In the typical notation that you will see in most articles
and books:

λ = c/ν

where:

λ = wavelengthν = frequencyc = speed of light

The speed of light is 299,792,458 meters per second (approximately
3.0 × 108 meters/second).
To be precise, what we usually call the "speed of light"
is really the speed of light in a vacuum (the absence of matter).
In reality, the speed of light typically varies depending
on the particular medium that it travels through. Light moves
more slowly in glass than in air, and in both cases the speed
is less than in a vacuum.

If we look at the colors of the rainbow from blue to red,
we can now understand that the blue light (400nm wavelength)
will have a higher frequency than red light at 700nm, with
the other colors of the rainbow falling in between. In fact,
the frequency of blue light will be 57% (400/700 × 100
= 57.14%) higher than the frequency of red light.

3) Energy:

As mentioned earlier,
light is a form of energy. According to the quantum theory,
all energy is transmitted and absorbed in discrete particles
called quanta or photons. Thus, the smallest amount of radiation
energy that can exist is one photon.

If one thinks of the photon as a small
packet or ball of energy, it is most useful in understanding
light, especially for our purpose of reefkeeping. For our
purposes, let us take a simplified, unified description that
says that light travels as discrete photons along a wave.
Visible light is a mixture of many photons with different
wavelengths. The photons are reflected and absorbed by various
surfaces, and when they reach the eyes, they create the sensation
of sight and resultant perceptions of color and brightness.
These photons are also directly responsible for photosynthesis
in plants and corals. The energy from the photons is used
during photosynthesis to convert CO2
into sugar, which is a primary energy source for the photosynthetic
endosymbiotic zooxanthellae living within corals.

As discussed earlier, the energy carried
by electromagnetic radiation is contained in the photons that
travel as a wave. According to quantum theory, the energy
in a photon varies with its frequency, according to the
equation:

As the frequency of the radiation increases
(wavelength gets shorter), the amount of energy in
each photon increases. Now we can begin to understand
why the red light gets absorbed quickly in water as a function
of depth.

These basic equations provide us with the relationship between
wavelength, frequency, energy and photons, and can be used
to go back and forth as seen in the following examples.

Example: What is the energy in a single photon of
light at 500nm?

E = 6.626 × 10-34
× 3.0 × 108/(500
× 10-9)
E = 0.039756 × 10-17
J

Example: How many photons per joule exist for light
at wavelength λ
= 500nm?

As seen above, to produce 1 Joule of energy by light at
a wavelength of 500nm requires a very large number of photons.
To avoid having to deal with such large numbers, we can measure
the number of photons in "moles" where 1 mole =
Avagadro's number = 6.02 × 1023.
So 25.15 × 1017 photons
would correspond to .000004177 moles. Now, this number is
too small, so instead we will measure in "micromoles,"
where 1 micromole (denoted as µmol) is 10-6
mole, giving us 4.177 micromoles of photons.

What about watts? Energy is measured in joules, and the "watt"
is the unit used as a measure of power. Power is defined as
the rate of flow of energy. By definition, 1 watt = 1 joule/second.
So, one watt of power from light at 500nm would need to provide
25.15 × 1017 photons
per second or 4.1769 micromoles/sec. The figure below shows
the relationship between watts and micromoles of photons to
generate 1 watt of power.

Summary:

This column has
focused on providing the basic terminology required to understand
light. Light is a form of energy, and can be simply described
as a stream of photons traveling along a wave. Photons are
discrete particles of energy. The characteristics of light
and the photons are specified by three terms: wavelength,
frequency and energy, which are mathematically related. Photons
with wavelengths of 400 nm carry more energy than those with
larger wavelengths and will appear violet to the human eye,
and photons with wavelengths of 700nm carry less energy and
will appear red. White light is a mixture of photons in the
wavelength range 400-700nm. This range is what the eye can
see and is also useful for photosynthesis. The photons carry
the energy and the number of photons is measured in units
of "micromoles."

The next column will discuss how light
sources generate photons, the distribution of the photons
in a light source and how this distribution is represented
as a spectral plot.

If you have any questions about this article, please visit my
author forum
on Reef Central.